I've built an oven myself from scatch using a relatively cheap method, which in short has a clay shell, with other layers of insulation over the top. I avoided the brick ovens because i didnt think i would be able to do the block work,

anyway...

my question is based on the finish coat of my oven which is a thin brushed on layer of sieved sand, cement and clay, with a little oxide for colour. I have since found out that this layer is not weather proof. when it rains, it seems to slowly wash off this thin layer.

can i use a comercial type sealer to possibly fix this problem? can you think of any problems that this may cause on my oven. ie chemicals near food??? is this a a big no no.

I'm using an acrylic stucco called, "Multitex". Like Les says, it's bulletproof!
It can be tinted to a suitable colour or it can be painted over with something like Dulux Tuscan or Solaguard Stone Finish.
I'm also in Oz, so I know you'll be able to get the products.

Try what they call flexible pointing. It is a flexible acrylic that will withstand the expansion created from the heat. It is hard to apply but I found it easier to thin it down with water and apply it like a thick paint, about three coats to give you about 3-4 mm total thickness. The stuff is designed for sealing terracotta or concrete ridge caps and is a standard roof tilers item. Wait until your oven is toally dry (say 5 cooking firings after your curing fires) You don't want to lock in the moisture.

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